Your might be changing the wrong one. The meshes used on PCOs are treated separately from staticobject meshes.
Try the "renderer.ambientColor' and/or the "renderer.diffuseColor" (you may need to change both).
Or as a last resort make a texture called defaultTexture.dds (make sure it has mipmaps and such when you save it!) and play around with that. DefaultTexture.dds is the file used for any object that lacks object light maps. Remove the old object light maps so that all objects will revert to DefaultTexture.dds. By default, this file is simply 50% gray. But you can make it darker and it should make ALL static objects appear darker! But ONLY those that have no objectlightmaps being applied to them!
Make sure that if you do the defaultTexture.dds method, that hte custome defaultTexture.dds is in the texture folder(NOT textures folder) of your map. Then add a alternate texture path for your map that points to this texture folder. That way only that map will have the new defaultTexture file. Because you don't want this to effect all your maps now do you?